1. Field of the Invention
The invention relates generally to metal halide lamps and more specifically to lamp igniters which do not emit high frequency acoustic noise when used with square-wave ballasts.
2. Description of the Prior Art
Igniters for metal halide lamps have been in existence for many years. Metal halide lamps between one and twenty kilowatts are used in motion picture movie sets. FIG. 1 shows a typical prior art lamp and igniter configuration that consists of: (a) a step-up transformer 10 that generates 5-15 KV from ordinary 120/240 VAC house current, (b) a high current, high frequency source obtained from a capacitor 12 and a pair of spark gaps 14 in combination, capacitor 12 is charged and discharged via spark gaps 14, and (c) a pair of linear transformers 16 to further boost voltages to the 40-80 KV levels necessary to start up a metal halide lamp 18. A square-wave ballast 20 and a capacitor 22 control the power delivered to lamp 18. Capacitor 22 protects ballast 20 from high voltage. To ignite lamp 18 a voltage is applied to transformer 10 resulting in a AC sine wave being output across capacitor 12. Spark gaps 14 fire near the peaks of the AC sine wave and produce high frequency on-and-off pulses in the primaries of transformers 16. High voltage develops across the secondaries of transformers 16, and is sufficiently high to ignite lamp 18. After ignition, transformer 10 is de-energized, and ballast 20 supplies operating power in the form of 50-400 Hz square-waves. Igniters are typically mounted in a box just below the lamp reflector head in one to twenty kilowatt metal halide lamps used in motion picture production.
Metal halide lamp igniters are typically mounted on the lamp heads themselves to avoid what would otherwise result in long runs of very hazardous high voltage cabling. Besides increasing the cost, long cabling attenuates the high frequency pulses present in the ignition voltage, and therefore tends to quench ignition. On a movie set, sound booms that carry microphones are often positioned around the action and near the metal halide lamp heads. The microphones are very sensitive and will pick up noise from many sources, including the lamp igniter nearby. A relatively new source of noise came into being when the traditional magnetic ballasts were replaced with electronic, or square-wave ballasts, in lamp igniters. The replacement had become necessary because magnetic ballasts caused the lamps to rapidly dim and brighten with each alternating cycle of the AC power. This caused a flickering in the film that is eliminated by synchronizing the shutter of the cameras to the line voltage. However, this isn't usually possible for video cameras, due to the broadcast frames-per-second standards that are in conflict. Besides being expensive, the synchronization technique was clumsy and very complex.
Square-wave power sources are preferred in metal halide lamp applications because, (1) no detectable modulation of the light output of the lamp will result, the modulation being particularly troublesome in motion picture and video lighting, and (2) electronic ballasts have a size and weight advantage over conventional magnetic ballasts, and yet produce such square-wave power. Because, an electronic ballast outputs square-waves, a fullwave rectification of the square-waves produces a current that will cause no noticeable flicker in unsynchronized cameras. However, square-waves coming from the ballasts through linear transformers, such as transformers 16, cause the cores within transformers 16 to sing, but only when power levels exceed approximately one kilowatt. Practical lamp building considerations limit maximum power levels to twenty kilowatts, so the singing transformer core problem exists only at one to twenty kilowatts.
The linear transformers commonly used in the prior art include ferrite or iron cores to improve transformer efficiency. As mentioned above, such cores have an undesirable habit of mechanically vibrating at high frequency (singing). This is due to an effect called magnetostriction. These vibrations are quite different from the sixty cycle hum and vibration common to large transformers and motors. In FIGS. 2-3, a prior art transformer 30 is comprised of a core 32, a primary winding 34, and a secondary winding 36. Core 32 is shown to be cylindrical in shape, but almost any shape is possible. Core 32 is commonly constructed of iron laminate or ferrite material. Such materials are more efficient than, e.g., solid iron, because energy robbing eddy currents are greatly reduced in the core. When a lamp's power source exceeds one kilowatt and consists of square-waves, acoustic noise is emitted by core 32. (High frequency excitation of the core is caused by the high frequency fourier components of square-wave AC.) The noise is relatively high frequency audio, being in excess of one kilohertz, and ranging up to the limits of human hearing, which is 15-20 KHz. Magnetostriction is believed to generate elastic contortions in the ferrite core that rapidly expand and contract the core's length and diameter, and may even cause the ferrite core to bend and flex along its length. Some experiments by the present inventor to reduce noise by dividing a ferrite core into several segments resulted in no decrease, as the individual pieces summed up with as much noise as was produced by a single piece.
Past commercial attempts to quash the noise created by transformer cores have either involved, (1) bypassing the igniter coils with a high voltage relay, and/or (2) eliminating the core altogether, thus eliminating the vibrating element. High voltage relays are not used very often because they are complicated and expensive. Air core transformers, especially at line frequencies, are inefficient and difficult to build. Without a core, a transformer must have a larger coil, and that increases the expense.
Several United States patents address the problems in building low-noise transformers in general. Prior art squelching of the noise created by transformer cores have almost universally addressed the problem of sixty cycle hum. The U.S. Pat. No. 4,724,413 issued to Kataoka discloses a low-noise transformer for use as an output transformer of an inverter. It is comprised of a sound-proof envelope surrounding at least one of the windings for absorbing or screening vibrating noise. The vibrations of the metal wall of a housing to which a ballast is mounted are suppressed in the U.S. Pat. No. 4,000,406 issued to Bhavsar by filling with potting compound. Stray magnetic fields, and therefor the vibrations caused by them, are controlled in the U.S. Pat. No 2,806,199 issued to Sola. Control of stray magnetic fields is obtained by increasing the cross-sectional area of the flux return path relative to the cross-sectional area of the flux generating path, thus reducing the effects of magnetostriction. Vibrations in a fluorescent lamp assembly can be reduced by snugly fitting magnetic sheets or plates about the windings of a ballast or inductor. This is described in U.S. Pat. No. 2,545,163 issued to Naster. The magnetic sheets define flux paths about the inductor and avoid creation of magnetic flux in the housings and reflectors of lamps. The U.S. Pat. No. 2,572,590 issued to Bjorklund surrounds an iron laminae magnetic core having windings with a "pulverous synthetic mass" and is placed in a heated press and baked under high pressure. Vibrations by the core are suppressed by making the transformer into one solid block. An improved method for impregnating a magnetic core with a potting compound using wax is disclosed in U.S. Pat. No. 3,160,840 issued to Lieberman. Mechanical humming is substantially reduced by making the cross-sectional area of an air gap larger that the cross section of an associated core in U.S. Pat. No. 3,391,366 issued to within a container for a magnetic core to rest against in U.S. Pat. No. 3,018,455 issued to Brandon Jr., et al. Unsaturated polyester and a polyurethane are used in combination to encase the core-and-coil elements to reduce mechanical noise in U.S. Pat. No. 3,683,303 issued to Ayamo, et al. The conversion of vibration to heat is improved when "d" loss factor values of the resinous materials are optimized. The U.S. Pat. No. 3,704,390 issued to Grahame discloses a magnetically permeable core member which is surrounded by the rolled turns of tape-like conductive foils and intervening layers of electrical insulation to produce sound deadening.